The aviation industry and climate change: what are contrails?  

A 2022 IPCC report found that direct GHG emissions from the transport sector accounted for 23% of global energy-related CO2 emissions in 2019. Road vehicles accounted for 70% of direct transport emissions, while 1%, 11%, and 12% of emissions came from rail, shipping, and aviation, respectively. 

As the mounting effects of climate change continue to be felt worldwide, the aviation industry is pioneering a method to reduce its contributions. Namely, it is focusing on efforts to curtail condensation trails – or contrails – which are fluffy, white cloud formations that sometimes appear as airplanes fly through the cold, humid, and icy parts of the atmosphere. Because they are a combination of soot, water vapor, and particulate matter (such as NOx), when aircrafts pass through these areas, they form cirrus clouds that absorb the radiation escaping from the surface, and, in turn, trap the heat. 

This phenomenon could account for around 35% of aviation’s total contribution to climate change — that’s about 1 to 2% of overall global warming! Together, these contrails roughly triple the total global warming impact of aviation compared to CO2 alone. Therefore, it is imperative that the aviation industry find solutions to reduce the production of contrails. 

What the industry has come up with: 3 solutions 

One method of reducing contrails consists of replacing traditional fuels with biofuels made from plant or animal biomass, waste, sugars and ethanol (corn). Sustainable jet fuels can produce 50%-70% fewer contrails according to research conducted by NASA and the German Aerospace Center (DLR). Jets using alternative fuels release fewer soot particles, thereby creating fewer ice crystal formations, which ultimately reduces contrail production by extension. Though biofuels may initially form larger crystals, they fall more quickly and melt in the warmer air below.

The second method involves developing electric or hydrogen-powered commercial aircrafts. Hydrogen is an attractive alternative to traditional aircrafts because it can be burned without emitting CO2 and is widely available. These aircrafts would either burn liquid hydrogen directly into their engines, or use gaseous hydrogen in a fuel cell system. With fuel cells, the hydrogen creates an electrochemical reaction that produces electricity to charge the aircraft's batteries while in flight. 

A third method involves redirecting flights to avoid contrail-inducing zones. Between 2% and 10% of all flights create around 80% of the contrails, so researchers have started developing predictive models that would allow airlines to identify and avoid contrail regions similarly to how they plan to avoid turbulence. The cost is predicted to be $0.5/ ton of CO2 equivalent. Furthermore, only minor adjustments to the routes of a small fraction of airplane flights is required, making predictive models highly attractive and cost effective. 

Some Challenges

While biofuels have great potential, they come with their own set of challenges. First is the issue of land use and its effects on agriculture. Producing three billion gallons of sustainable aviation fuel would require between 8 and 11 million acres of corn or 35 and 50 million acres of soybeans, depending on crop yields. This could impact food production and cost. Shifting to corn or soybean based fuels has also been found to produce significant adverse emissions impacts. Lastly, it’s unclear whether sustainable fuels can meet the world’s growing demand for aerial transportation.   

While hydrogen is attractive, it has lower energy density than fossil fuels, meaning that a higher onboard fuel storage volume is needed to cover the same distance as current fossil fuel-powered aircrafts. In addition, H2-powered large passenger planes would require significant changes to aircraft design, making it less cost effective in the short term when RD&D costs are considered (development of fuel cell technology and liquid hydrogen tanks, aircraft research, hydrogen infrastructure, fleet output, etc). Industry experts anticipate that it will take 10 to 15 years to make these important advancements. 

Lastly, contrail prediction models rely on a variety of input data, including flight trajectories, aircraft and engine parameters, fuel characteristics, and weather data. However, the availability and accuracy of some of these data inputs is still a challenge, as no standardization exists. 

Who is our guest? 

Matteo Mirolo is Head of Policy and Strategy, Contrails at Breakthrough Energy, an organization founded by Bill Gates to spur innovation in clean energy and address climate change. Prior to that he was sustainable aviation policy manager at Transport & Environment (clean transport advocacy group). Mirolo is also a member of the sustainability advisory panel at Air New Zealand. 

Resources

• IPCC Sixth Assessment Report: Transport
• The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018
• Biofuels
• NASA-DLR Study Finds Sustainable Aviation Fuel Can Reduce Contrails
• Hydrogen could power the next-gen aircraft of tomorrow
• Land-Use Impacts of the Sustainable Aviation Fuel Grand Challenge
• How much biofuel would we need to decarbonise aviation?
• Hydrogen-powered aviation

Further reading

• Aviation Contrails 
• The missing policies on aviation emissions 

For a transcript of this episode, please visit https://climatebreak.org/eliminating-contrails-to-increase-aircraft-sustainability-with-matteo-mirolo/.